As used herein, the term asphalt also comprises macadam and tarmac. Asphalt paved road surfaces typically comprise a mixture of asphalt cement (typically a black, sticky, petrochemical binder) and an aggregate comprising appropriately sized stones and/or gravel. The asphalt concrete mixture is usually laid, compressed and smoothed to provide an asphalt paved road surface.
Over time, an asphalt paved road surface can deteriorate as a result of a number of factors. For example, seasonal temperature fluctuations can cause the road surface to become brittle and/or cracked. Erosion or compaction of the road bed beneath the road surface may also result in cracking. Moreover, certain of the chemical constituents incorporated in fresh asphalt are gradually lost over time or their properties changed with time, further contributing to brittleness and/or cracking of the road surface. Where concentrated cracking occurs, pieces of pavement may become dislodged. This dislodgement can create traffic hazards, and accelerates the deterioration of adjacent pavement and highway substructure. Even if cracking and the loss of pavement pieces do not occur, the passage of traffic can polish the upper highway surface, and such a surface can be slippery and dangerous. In addition, traffic-caused wear can groove, trough, rut and crack a highway surface. Under wet highway conditions, water can collect in these imperfections and set up dangerous vehicle hydro-planing phenomena. Collected water also contributes to the further deterioration of the pavement.
Prior to about the 1970's, available methods for repairing old asphalt-paved road surfaces included: spot treatments such as patching or sealing, paving with new materials over top of the original surface, and removal of some of the original surface and replacement with new materials. Each of these methods had inherent drawbacks and limitations.
Since about the early 1970's, with increasing raw material, oil and energy costs, there has been a growing interest in trying to recycle the original asphalt. The world's highways have come to be recognized as a very significant renewable resource.
Early recycling techniques involved removing some of the original surface and transporting it to a centralized, stationary recycling plant where it would be mixed with new asphalt and/or rejuvenating chemicals. The rejuvenated paving material would then be trucked back to the work site and laid. These techniques had obvious limitations in terms of delay, transportation costs and the like.
Subsequently, technology was developed to recycle the old asphalt at the worksite in the field. Some such processes involved heating and are frequently referred to as "hot-in-place recycling" (hereinafter referred to as HIPR).
This technology comprises many known processes and machines in the prior art for recycling asphalt paved surfaces where the asphalt has broken down. Generally, these processes and machines operate on the premise of (i) heating the paved surface (typically by using large banks of heaters) to facilitate softening or plasticization of an exposed layer of the asphalt; (ii) mechanically breaking up (typically using devices such as rotating, toothed grinders; screw auger/mills; and rake-like scarifiers) the heated surface; (iii) applying fresh asphalt or asphalt rejuvenant to the heated, broken asphalt; (iv) distributing the mixture from (iii) over the road surface; and (v) compacting or pressing the distributed mixture to provide a recycled asphalt paved surface. In some cases, the heated, broken material can be removed altogether from the road surface, treated off the road surface and then returned to the surface and pressed into finished position. Much of the prior an relates to variations of some kind on this premise.
Over time, HIPR has had to address certain problems, some of which still exist today. For example, asphalt concrete (especially the asphalt cement within it) is susceptible to damage from heat. Thus, the mad surface has to be heated to the point where it was sufficiently softened for practical rupturing, but not to the point of harming it. Furthermore, it was recognized that asphalt concrete is increasingly hard to heat as the depth of the layer being heated increases. Many patents have attempted to address these problems.
U.S. Pat. No. 3,361,042 (Cutler) discloses a process for wad surfacing. The process comprises the steps of: heating the road surface in a non-oxidizing environment; scarifying the heated surface deeply; piling the scarified material in windrows; heating the piled windrows in a non-oxidizing atmosphere; initially planing, levelling and kneading the heated mixture; adding minor mounts of conventional tack coat; finally planing, levelling and kneading the mixture; tamping and screeding the mixture; and compacting the mixture. The steps of initial and final planing, levelling and kneading of the mixture may be repeated during or omitted from the process.
U.S. Pat. No. 3,970,404 (Benedetti) discloses a method of reconstructing asphalt pavement. Generally, the method comprises heating the asphalt surface in successive stages during timed intervals. This gradual heating apparently permits the heat to penetrate the asphalt more deeply with minimal or no overheating thereof. The heated asphalt is then scarified to a depth not greater than that to which it has been heated. The scarified asphalt is then worked to provide a recycled asphalt surface. This method is somewhat inefficient since scarifying is effected only when the heat has penetrated the asphalt surface to a desired depth. As is well known in the art, in certain instances, the depth of heat penetration is directly related to the square root of the time provided for heat penetration, i.e.--25 seconds may be required for the heat to penetrate to a depth of 5 millimeters while 49 seconds may be required for the heat to penetrate to a depth of 7 millimeters. Thus, increasing the time allowance for desired heat penetration results in a decrease in overall process efficiency.
U.S. Pat. No. 3,843,274 (Gutman et al) discloses an asphalt reclaimer. Generally, the reclaimer is adapted to carry out the following steps: heating the asphalt surface, cutting the heated surface, conveying the cut surface away from the road to a pugmill, pulverizing the cut surface in the pugmill, redistributing the pulverized asphalt back onto the road surface and levelling the redistributed asphalt to provide a recycled asphalt surface.
U.S. Pat. No. 3,989,401 (Moench) discloses an apparatus for renewing or reconditioning asphaltic pavement surfaces. Generally, the apparatus comprises a hood and burner assembly which heats a surface over which it is moved, a scaring assembly that scrapes, breaks up and distributes the heated surface material and a levelling assembly that levels the scarified surface and material. This reference does not disclose or suggest processing of the scarified material to rejuvenate it in place.
U.S. Pat. No. 4,011,023 (Cutler) discloses a machine for recycling macadam highway pavement. The subject machine is intended to be used on a pavement surface which has been previously scarified or dislodged. This loose material is removed from the road surface, thereafter heated, mixed with fresh asphalt and spread on the original roadbed site. Heating is conducted off the road surface in a special chamber using a complicated multi-directional conveyor system. This machine is cumbersome and deficient since it requires complicated and expensive conveyors to remove the surface to be recycled from the road, to heat the removed material and reapply it thereafter.
U.S. Pat. No. 4,124,325 (Cutler) discloses a method and apparatus for recycling asphalt concrete roadways. Essentially, the process comprises heating the pavement surface with propane fired emitters; scarifying the heated surface to penetrate and excavate the entire surface to a depth of approximately 3/4 inch; applying asphalt over the heated, scarified surface; mixing the excavated material; commingling the excavated material with additional hot mix in a pugmill rotor; and levelling the mix from the pugmill rotor on the roadway to provide a recycled asphalt surface.
U.S. Pat. Nos. 4,129,398 and 4,335,975 (both to Schoelkopf) disclose a method and apparatus for plastifying and tearing up of damaged mad surfaces and covers. The method comprises plastifying (heating) and breaking up the road surface with first and second separate and distinct devices. The second device aim serves the purpose of distributing, rearranging and profiling the broken-up material on the road surface in the absence of fresh asphalt being applied to the road surface. Thereafter, a third separate and distinct device is used to apply fresh asphalt or other bituminous material onto the broken-up, distributed, rearranged and profiled top surface of the road.
U.S. Pat. No. 4,226,552 (Moench) discloses an asphaltic pavement treatment apparatus and method. Generally, the method comprises heating and scarifying the asphalt surface to form a loose aggregate-asphaltic mixture on the ground surface. This mixture is then removed from the ground surface, heated, thoroughly mixed with a conditioner for the asphalt and reapplied to the ground surface as a mat. This method is inefficient since each treatment is carried out by an independently operable, portable apparatus and since the asphaltic must be removed from the road surface for reconditioning.
U.S. Pat. No. 4,534,674 (Cutler) discloses a dual lift repaving machine. The machine includes, in series: a preliminary heater; a preliminary scarifier; a main heater; a main scarifier; a sprayer for spraying liquid asphalt cutback onto the heated, scarified road surface; a first macadam dispensing device to dispense hot mix onto the sprayed, heated, scarified road surface; a first mixer for commingling the hot mix and the sprayed, heated, scarified road surface; a first screed to level and partly compact the material to form the first lift; a second macadam dispensing device to dispense additional hot mix onto the road surface; a second mixer for mixing the hot mix in situ; and a second screed to level and compact the new hot mix to provide a second road lift. The necessity of providing two lifts renders this machine complicated to use and relatively expensive to acquire.
U.S. Pat. No. 4,545,700 (Yates) discloses a process for recycling asphalt pavement. Essentially, the process purports to overcome the difficulties associated with inefficient heat penetration into the asphalt surface by providing steps of serially heating and milling multiple layers of the asphalt surface until the desired depth of asphalt has been removed and then, optionally, mixing the heated asphalt with additives. Typically, each heating/milling step results in removal of a strip which is at least 1/4 inch deep. This process requires the use of many heaters and millers which are complicated and expensive machines.
U.S. Pat. No. 4,711,600 (Yates) discloses a heating device for use with asphalt pavement resurfacing equipment. The only example of resurfacing equipment disclosed is an apparatus in which layers of the road surface are successively heated, milled and removed from the road surface, via conveyors, for mixing with fresh asphalt or asphalt rejuvenant, and subsequent reapplication to the road surface. The use of a plurality of conveyors can be problematic since it adds excessive cost and complexity to the task at hand.
U.S. Pat. No. 4,784,518 (Cutler) discloses a double-stage repaving method and apparatus. The subject method includes a first stage comprising the steps of: heating an upper layer of an asphalt surface; scarifying the heated upper layer; adding recycling agent to the upper layer and thoroughly mixing and screeding the mixture to form recycled material; and adding fresh asphalt to the recycled material and milling the combination to form a mixed material thereby leaving exposed a lower layer of asphalt material. The second stage in the method comprises: conveying the mixed material from the first stage away to a paving station at the end of the process; subjecting the exposed lower layer of asphalt material to the same heating, scarifying, treatment and working steps to which the upper layer was subject; and laying the mixed material down on the exposed road surface (i.e. upper and lower asphalt layers removed) to provide a recycled road surface. This method is deficient as it requires the use of two relatively expensive and complicated conveyors.
U.S. Pat. No. 4,793,730 (Butch) discloses a method and apparatus for asphalt surface renewal. Generally, the method comprises the steps of: steam heating the asphalt surface; breaking the heated surface to a depth of about two inches and thoroughly mixing in situ lower material in the asphalt with the broken material; further steam heating the material to fuse the heated mixture into a homogeneous surface; screeding the homogeneous surface; and compacting the screed surface. The method and apparatus purportedly can be used to resurface asphaltic paving surfaces without requiting the addition of new materials or rejuvenants.
U.S. Pat. No. 4,929,120 (Wiley et al) discloses a two-stage process for rejuvenating asphalt-paved wad surfaces. In the first stage of the process, the entire width of the original asphalt surface is heated to a depth of about 1 inch and a temperature of about 300.degree. F. The hated upper surface is then removed completely from the wad surface (using scarifying, windrowing and conveying techniques) to expose a lower asphalt surface corresponding to the entire width of the original asphalt surface. In the second stage of the process, the lower asphalt surface is heated to a depth of about 1 inch and a temperature of about 300.degree. F. The heated lower surface is then ruptured (e.g. scarified) and either left in place or completely removed from the wad surface. If the ruptured lower surface is left in place, asphalt from the upper layer and, optionally, fresh asphalt (or asphalt rejuvenant is applied thereover. Alternatively, if the ruptured lower surface is completely removed it may be commingled with asphalt from the upper layer and, optionally, fresh asphalt (or asphalt rejuvenant), and thereafter returned to the wad surface. Finally, pressure is applied to force the upper/lower layer mixture against the road surface to provide a smooth, recycled surface. This process is somewhat deficient since it requires removal of at least the upper portion of the asphalt surface necessitating the use of relatively expensive and complicated equipment.
U.S. Pat. No. 4,850,740 (Wiley) discloses a method and apparatus for preparing asphaltic pavement for repaving. This patent purportedly provides an improvement over U.S. Pat. No. 4,929,120 by eliminating the need to remove the upper layer of heated, scarified asphalt completely away from the wad surface prior to treatment of the lower layer of asphalt. Essentially, the improvement relates to heating, scarifying and windrowing the asphalt surface in a manner to provide a central strip comprising windrowed material from outer strips of the asphalt surface piled onto an untreated (i.e. not scarified/removed) central strip of the asphalt surface. The central strip is then ground to mix the centrally windrowed material with the previously unground central strip of the asphalt surface. This mix is then spread over the entire asphalt surface and pressed into place. This proems is somewhat deficient since it requires two separate and distinct grinding steps.
It would be desirable to have a method and apparatus for recycling asphalt surfaces which method and apparatus overcome or reduce at least one of the above-identified disadvantages of the prior art.